1. Field of the Invention
The present invention relates to a steering column assembly including a telescopic adjustment mechanism which allows a driver to adjust the position of a steering wheel depending on the driver's physique and preference in order that the driver can feel comfortable while driving.
2. Description of the Related Art
Known is a steering column assembly including a tilt adjustment mechanism and a so-called telescopic adjustment mechanism. The tilt adjustment mechanism allows a driver to adjust the height of the steering wheel depending on the driver's physique and driving posture. The telescopic adjustment mechanism allows the driver to adjust the front-back position of the steering wheel in the axial direction of the steering shaft depending on the driver's physique and driving posture.
In general, when a telescopic adjustment is applied to the steering wheel, the steering column assembly of this type causes an eccentric cam to be pressure-contacted to and engaged with an inner tube constituting the steering column, the eccentric cam being attached to the outer periphery of a clamp bolt to rotate integrally with the bolt. Thereby, the inner tube is pressed against the inner periphery of an outer tube, and thus the inner tube is locked to the outer tube. This locking mechanism eliminates play between the inner tube and the outer tube (space between the inner tube and the outer tube in the radial direction). (See Japanese Patent No. 3783524, for example.)
However, the technique disclosed in Japanese Patent No. 3783524 has a problem of poor operability for the following reason. The shank of the clamp bolt constituting the rotary shaft of the eccentric cam is arranged outside the outer tube, so that the space between the shank and the inner tube should be set wide. In addition, the radial dimension of the eccentric cam (the dimension of the eccentric cam from its rotation center to its outer circumferential surface) is relatively large. Consequently, the clamp bolt needs a large clamping torque to obtain a force with which the outer circumferential surface of the cam presses the inner tube, when the inner tube is locked to the outer tube by the rotation of the eccentric cam. Accordingly, the operation lever has to be operated with a larger force.
Furthermore, when a force is applied to the steering column in a direction in which the steering column contracts while the eccentric cam is locking the inner tube to the outer tube, it is likely that the inner tube biases the eccentric cam with a friction force of the inner tube in a direction in which the inner tube is released from the outer tube. In this case, the eccentric cam may rotate, and the inner tube may be accordingly released from its locked condition, so that a so-called “operation lever return” may occur.